1. Field of the Invention
This invention relates to a motor control apparatus for controlling a motor.
2. Description of the Prior Art
A motor control apparatus in which a rotational speed of the motor is detected to control the electrical power supplied to the motor in response to the detected signal thus obtained has been widely applied to a capstan motor or the like of a video tape recorder (hereinafter abbreviated as a VCR).
On the other hand, VCR units are always required to be reduced in size and weight. In order to meet these requirements, miniaturization of the mechanism has progressed. For this, the capstan motor is downsized as well, and the moment of inertia largely reduced. The reduction of the moment of inertia of a motor may result in a degradation in control performance. A control apparatus which reduces the rotational speed variation of such a motor has been investigated previously. For example, an iterative control method using a memory is disclosed in Japanese Laid-Open Patent Application No.5-10032. According to this method, the speed variation for any disturbance torque which is an integral number of times a specific frequency can be largely reduced. In addition, control methods using a disturbance observer have been proposed. The use of the disturbance observer makes it possible to reduce the speed variation for the disturbance torque at low frequency.
However, previous investigations were made mainly on how to reduce the rotational speed variation under a steady-state condition, and detailed investigations were not made on the operational process of the motor from the starting to the steady-state condition. Here, a general method of starting a conventional motor will be explained. Recently, VCR and the like have been frequently controlled by software using a microprocessor, however, the speed control is generally carried out at a time point when a detected signal of the rotational speed is inputted. As a result, if the motor is stopped and a detected speed signal is not inputted, the speed control is not carried out. As a result, in general, time measurement is carried out using a timer, and when the detected speed signal is not inputted for a predetermined time interval, a speed control is carried out so that the motor generates the maximum acceleration which is determined in accordance with the specification of the motor. Such a process is hereinafter called maximum acceleration processing. And, if the speed detection was made, a speed error is calculated from the speed detected signal and a control signal of the motor is generated so as to be proportional to the speed error. In addition, in order to suppress the steady-state speed deviation, an integral compensator is provided through which the speed error is integrated and a signal proportional to the value thus obtained is added thereto.
Then, in the VCR case, it is necessary to realize special reproducing functions in addition to the normal recording and reproducing functions. Typical one of the special reproducing functions is a slow reproduction function. Here, with the VCR performing multi-segment recording (for example, the digital VCR having an increased information amount, in order to perform the slow reproduction, a method will be considered in which a tape is run intermittently every one frame (or every one field). In this case, a method is used in which the tracking of a recording track and the head is not performed and the data is reproduced by scanning the same track several times. Namely, the tape is intermittently run at a low speed. In this case, however, if a motor of low inertia is run intermittently at a low speed under the large load condition, the speed control operation will become unstable with the methods shown above.
Since the response frequency of the the integral compensator cannot be set high to secure the stability of the control system, the time required for the response to converge becomes long. Accordingly, the integral compensator has no effect when starting the motor. By the maximum acceleration processing when starting the motor as shown above, the generated acceleration is large due to the fact that the moment of inertia of the motor is small and as a result, the rotational speed of the motor reaches the targeted value before the speed is detected (the speed detection is performed such that the motor is rotated for a constant time interval and the time required therefor is measured). In this case, the speed error becomes zero, and if the integrator is not provided, the control signal outputted from the control apparatus becomes one equivalent to the torque-off condition. In such case, if the load applied to the motor is large, the motor will be stopped. In addition, if the maximum acceleration processing is performed, the motor is operated so as to be intermittently started and stopped, resulting in the vibratory condition.